Purpose The purpose of this paper is to evaluate differences between IOL master and A-scan regarding axial length (AXL) and predicted IOL power in different types of cataract.Patients and methods Forty eyes of 40 patients underwent examination by IOL master and A-scan, where average K-reading, AXL and predicted IOL power were compared.Results Forty eyes of 40 patients were included. The mean AXL measured by IOL master was higher (26.18±2.92 mm) than that with A-scan (26.02±2.99 mm) with a mean difference of 0.2±0.44 mm (P=0.07). The mean predicted IOL power was 11.61±8.33 D with IOL master versus 12.01±8.23 D with A-scan (P=0.03). However, no statistically significant difference was found regarding average K-readings and predicted postoperative refraction (P=0.4 and 0.7, respectively). Bland–Altman plots showed almost perfect agreement between both methods regarding AXL and predicted IOL power. Further subgroup analysis revealed a statistically significant difference in AXL between both devices only in nuclear cataract with no significant difference in cases with complicated cataract to myopia or silicone oil (P=0.013, 0.2 and 0.1, respectively). No statistically significant difference was found between the three groups regarding the calculated IOL power (P=0.34, 0.13 and 0.15, respectively). Bland–Altman analysis showed almost perfect agreement for the mean difference of AXL and IOL power in the three subgroups.Conclusion There is no significant difference between IOL master and A-scan biometry, with the noncontact IOL master being preferred by patients; however, there exists certain situations where A-scan is still mandatory.

Accurate calculation of the intraocular lens (IOL) power necessary for attaining the desired postoperative refraction is still a research issue which depends on several factors, including axial length (AXL), keratometry, anterior chamber depth and lens formulas. Of these factors, the preoperative measurement of AXL is considered to be a key determinant in calculating the IOL power to be implanted [1].

Studies based on preoperative and postoperative ultrasound biometry show that 54% of errors in predicted refraction after IOL implantation can be attributed to AXL measurement errors, 8% to corneal power measurement errors and 38% to incorrect estimation of postoperative anterior chamber depth [2].

Previously, applanation ultrasound (A-scan) biometry has been the most commonly used technique for AXL measurement. More recently, partial coherence laser interferometry has gained more preference [3].

The AXL when measured by applanation A-scan ultrasound causes erroneous AXL measurement and an undesired postoperative refractive outcome. This might be attributed to the indentation of the globe and an off-axis measurement of the AXL by the transducer particularly important in highly myopic eyes [1].

IOL master is a fast, noncontact method reported as a potentially more accurate method than ultrasound biometry [4]. IOL master uses the method of partial coherence interferometry (PCI) to measure the AXL, based on reflection of the interference signal of the retinal pigment epithelium. This technique was found to be more accurate than the acoustic method in cataractous eyes, with no other pathologies [5]. It has also been suggested that the IOL master is more precise and useful in difficult situations, including high myopia, posterior staphyloma or silicone oil (SO)-filled globes [6].

The AXL measurement with the IOL master is not affected by the subjective error sources of acoustical A-scan ultrasound biometry. Measurement along the visual axis is ensured as the patient fixates on the light source, precluding a misalignment error produced by an off-axis posterior staphyloma [7]. However, it will not work in the presence of significant axial opacities. A mature or darkly brunescent lens, dense posterior subcapsular plaque, vitreous hemorrhage or central corneal scar will preclude any type of meaningful measurement [5]. On the other hand, eyes with posterior staphylomata, or eyes with SO, are very easy, and routinely measured with the IOL master [8].

Success in visual improvement in SO-filled phakic-induced cataractous eyes that require oil and/or cataract removal, and IOL implantation in one operation, depends on an accurate AXL measurement and a precise IOL power calculation. However, biometry in SO-filled eyes is difficult to perform and measurement may be unobtainable, due to sound attenuation. Using A-scan ultrasound biometry in SO-filled eyes has several fallacies, such as false longer eyes, presence of multiple fluid interfaces, or poor penetration from sound absorption by oil [9].

IOL master had more accuracy and less deviation in predicting postoperative refractive error than A-scan immersion in SO-filled eyes [10].

The aim of this work was to determine whether IOL power calculations for cataract surgery as measured by postoperative refractive error using PCI are more accurate in improving postoperative outcomes than applanation ultrasound biometry in high myopic patients, SO-filled eyes, and in nuclear cataracts (I and II).

Patients and methods

This study was conducted on 40 eyes of 40 patients (18 men and 22 women) during the period between March 2014 and August 2014. The study patients were recruited from patients scheduled for phacoemulsification and IOL implantation.

The study patients who underwent routine ophthalmologic examination were informed about the purpose of the study and had to give an informed consent before inclusion. Data collection conformed to all local laws and was compliant with the principles of the Declaration of Helsinki. The study was approved by the Ethics Committee of Ophthalmology Department, Cairo University.

Eyes with no ocular pathology apart from age-related cataract or complicated cataract to silicone injection or myopia were included in the study. The patients were divided into three groups including patients with SO-filled eyes who were scheduled for SO removal and phacoemulsification, high myopia with spherical equivalent (SE) greater than or equal to −6 D and or AXL greater than or equal to 26.0 mm, and eyes with nuclear cataract I and II.

The study excluded patients having retinal detachment, eyes whose AXL could not be measured with IOL master, because of dense ocular media opacities such as corneal scars or dense posterior subcapsular cataracts, and patients who underwent previous corneal surgery (including refractive surgery). Patients having intraoperative complications as inability to achieve secure ‘in the bag’ placement of the IOL (i.e. due to posterior capsule rupture, vitreous loss, weak zonules, or zonular rupture) were also excluded from the study.

Keratometry-readings (K) were obtained by an automated keratometer and used by the A-scan ultrasound in IOL power calculation.

AXL and IOL power calculation were obtained with the IOL master, followed by ultrasound biometry on the same day, consecutively by the same investigator.

AXL measurements were first performed by IOL master (AXLm) followed by applanation ultrasonography (AXLUS). This order was considered necessary to maintain the integrity of the corneal epithelium, which may be compromised inadvertently by its contact with the ultrasound probe. Ultrasound biometric sound velocities of 1532 m/s were taken for the aqueous and the vitreous humor and 1641 m/s for the lens. Five AXL measurements were obtained by applanation ultrasonography and a mean of at least three valid measurements was used as the AXL. In SO-filled eyes, the velocity of the beam was changed to 980 m/s to eliminate the magnified AXL induced by the presence of SO.

Measurements were taken with the patient sitting upright and the transducer held so that the ultrasound beam was perpendicular to the globe. This position helps in keeping any fluid downwards and silicone in touch with the central retina so that no pocket of fluid can affect the measurement taken.

The IOL master used in AXL measurement is based on the principle of dual beam of PCI. It uses infrared light (λ=780 nm) of short coherence for the measurement of the optical AXL, which is converted to geometric AXL by using a group refractive index. The calculation for SO-filled eyes is already integrated into its program. Eyes with signal-to-noise ratio less than 1.6 were excluded from the study. Signal-to-noise ratio is an indicator of the quality of the AXL measurement. It is automatically assessed during internal calculation of the AXL from the interface signal. The measurements were taken with the patient in a sitting position using ‘phakic SO-filled eyes mode’.

The theoretic IOL power prediction formula (SRK/T) was used for all calculations, aiming for postoperative emmetropia in all eyes.

All patients underwent phacoemulsification and in the bag IOL implantation through a temporal clear corneal incision. SO removal was performed through a posterior approach (sutureless 23-G sclerotomies). The IOL power was determined according to the optical biometry results aiming for emmetropia ranging from −0.50 to +0.50 D.

At last follow-up visit, ∼1 month following the operation SE using auto refraction (AutoRef-Keratometer RK-3, Canon, Japan) and subjective manifest refraction were performed by the same examiner and best-corrected visual acuity (BCVA) was tested using a Snellen chart.

Data analysis was performed using the statistical package SPSS (SPSS Inc., Chicago, Illinois, USA), version 21. The data was summarized using mean±SD, minimum and maximum in quantitative data and using frequency (count) and relative frequency (percentage) for categorical data. The mean absolute error (MAE) was calculated as the absolute difference between measured error and the predicted error for each method. Comparisons between variables measured using the IOL master and A-scan were done using paired t-test in normally distributed data, while nonparametric Wilcoxon’s test was used for non-normal distributed data. Correlation was done to test for linear relations between quantitative variables measured by IOL master and A-scan by Spearman’s correlation coefficient (r). P-values less than 0.05 were considered as statistically significant. Interdevice correlation was evaluated by Pearson’s moment correlation coefficient (r). Finally, interdevice agreement was evaluated by the method described by Bland–Altman. With this method, intermeasurement differences were plotted against their means and the 95% limits of agreement (LoA) were determined (95% LoA=mean interdevice difference±1.96 SD). These plots are useful for examining any systematic bias − the amount by which one instrument consistently overestimates the other − as well as the variability about this difference (the spread of errors).

Results

Forty patients (40 eyes) were included in the study (18 men and 22 women) with a male: female ratio of 1 : 1.22.

The mean age of the study group was 55.52±10.5 years (range: 21–71 years), with a mean preoperative SE of −6.01±6.82 D (range: −18.13 to +9.75 D).

The types of cataract were subgrouped into age-related nuclear cataract in 15 (37.5%) cases; complicated cataract to SO in 10 (25%) cases and complicated cataract to high myopia in 15 (37.5%) cases.

K-readings were obtained by the automated keratometer and IOL master while AXL measurements were first performed by the IOL master (AXLm) followed by applanation ultrasonography (AXLUS) as demonstrated in [Table 1] and [Figure 1].

There was a statistically significant difference between both devices regarding the measured AXL as well as the calculated IOL power (P=0.07 and 0.03, respectively). However, no statistically significant difference was found regarding the average K-readings and the predicted postoperative refraction (P=0.4 and 0.7, respectively).

Agreement between both devices regarding the AXL and the estimated IOL power was analyzed using the Bland–Altman method with 95% LoA as demonstrated in [Figure 2], where both devices show almost perfect agreement (Cronbach’s α=0.995 for the mean difference of both parameters). Linear regression analysis showed no proportional bias (t-score=−1.07, P=0.3 for the AXL) and (t-score=0.58, P=0.56 for the IOL power) ([Figure 3]).

Figure 2 Bland–Altman analysis for agreement between the IOL master and A-scan measurements as regards axial lengths (AXL) (top) and IOL power (bottom).

Further subgroup analysis revealed a statistically significant difference in the AXL between both devices only in nuclear cataract with no significant difference in AXL in cases with complicated cataract to myopia or SO (P=0.013, 0.2 and 0.1, respectively). No statistically significant difference was found between the three groups regarding the calculated IOL power (P=0.34, 0.13 and 0.15, respectively).

Bland–Altman analysis showed almost perfect agreement (Cronbach’s α=0.9) for the mean difference of AXL and IOL power in the three subgroups.

Postoperative BCVA was 6/9 or better in 80% of the eyes, 6/12 in 12.5% of the eyes and 6/18 in 7.5% of the eyes. Postoperative SE in 37 patients ranged from −0.50 to+0.50 D and was −0.75 D in three patients. One hundred percent of the patients were within −0.75 to +0.75 D with an overall accuracy of 92.5% ([Table 2]).

The mean predicted errors measured using IOL master was − 0.11±0.18D (range: − 0.57 to +0.18 D), while the mean measured postoperative SE was –0.26±0.27 (range: −0.75 to +0.25), the mean difference between the predicted error and the postoperative SE was 0.16±0.3 (P=0.003).

Mean numerical errors (MNE) measured by the IOL master was −0.16±0.32 (range: −0.91 to 0.57). MNE measured by A-scan was −0.17±0.43 (range: −1.29 to 0.61). The mean difference between the two methods was 0.016 (P=0.671), which is statistically nonsignificant.

MAE measured by IOL master was 0.26±0.24 (range: 0.01–0.91). MAE measured by A-scan was 0.37±0.28 (range: 0.00–1.29). The mean difference between the two methods was −0.11±0.2 (P=0.001).

Being a noncontact method, all patients preferred the IOL master to the applanation biometry.

Discussion

The IOL master utilizes a noncontact technique for AXL measurements which measures the distance between the anterior corneal surface and the retinal pigment epithelium. On the contrary, A-scan biometry measures the distance from the corneal vertex to the internal limiting membrane internal limiting membrane (ILM). Due to the thickness of the cell layer, the resulting differences of the measured AXL are between 150 and 350 μm [11],[12].

IOL master biometry has greater accuracy than ultrasound biometry because it measures the ocular AXL along the visual axis, as the patient fixates at the measurement beam, whereas during ultrasound biometry a misalignment between the measured axis and the visual axis may occur [12].

The error in measurement is theoretically more obvious in highly myopic eyes, which have a long AXL and low scleral rigidity. The AXL measurement will be inaccurately shorter with corneal indentation in highly myopic eyes. Posterior pole staphylomas in eyes with an extremely long AXL can also lead to errors in A-scan AXL measurement. IOL master in posterior pole staphyloma gives better results because of the more precise localization of the fovea [11].

In this study, the AXL measured using the IOL master was significantly longer by 0.2 mm than the AXL measured by A-scan. Previous studies agree with this finding [8],[13].

In this study, the mean predicted IOL power was significantly less using the IOL master with a mean difference of −0.4. This is consistent with previous study by Eleftheriadis [8] who also used different formulae for IOL power prediction.

There was no statistically significant difference regarding the mean predicted error in IOL power calculation (the mean difference was −0.016).

Postoperative BCVA was 6/9 or better in 80% of the eyes, 6/12 in 12.5% of the eyes and 6/18 in 7.5% of the eyes. The postoperative mean SE error was −0.26±0.27 D (range: −0.75 to +0.25 D). In 12.5% of the 40 patients (five patients), a 0.00 postoperative refraction was achieved. Postoperative SE error in 37 patients ranged from −0.50 to +0.50 D and was −0.75 D in three patients. In all patients the SE was within ±0.75 D. Thus, the overall accuracy was 92.5%.

The difference in MNE was not found to be statistically significant; however, MNE suffers from the disadvantage of averaging both positive and negative errors. The MAE difference between ultrasound and the IOL master, a more useful measure of the true size of the error, was statistically significant improving a 0.37 D error to 0.26 D. This reflects a 30% improvement in absolute postoperative refractive error with the IOL master compared with applanation ultrasound using SRK/T lens formula. Rose and Moshegov [13] reported similar results where MNE difference was not found to be statistically significant, while the MAE difference between ultrasound and the IOL master was statistically significant improving a 0.65 D error to 0.42 D. This reflects a 35% improvement in absolute postoperative refractive error with the IOL master compared with applanation ultrasound using SRK/T lens formula. A similar study by Findl et al.[12] using the SRK II formula reported a 27% improvement with PCI. This study found ±1 D error with ultrasound of 72.9% was improved to 85% and ±2 D error with ultrasound of 96.4% was improved to 100% with interferometry.

There was a statistically significant difference between both methods when comparing the predicted IOL power to the postoperative SE.

No statistically significant differences exist between both methods in different types of cataract regarding AXL measurement and IOL power calculation except for estimating AXL in nuclear cataracts, where the AXL is significantly longer with the IOL master (mean difference=0.12).

The IOL master has simplified considerably the process of ocular biometry. It is a noncontact technique, which does not require the use of topical anesthesia, thus providing comfort to the patient and preventing corneal abrasions and the transmission of infections. Furthermore, it has greater accuracy than ultrasound biometry because it measures the ocular AXL along the visual axis, as the patient fixates at the measurement beam, whereas during ultrasound biometry a misalignment between the measured axis and the visual axis may result in erroneously longer AL measurements. This is especially important in eyes with posterior pole staphyloma, because of the more precise localization of the fovea. In addition, it is easier to master its use.

However, IOL master is limited by its inability to measure AXL in dense ocular media, nonoptimal fixation as in cases of age-related macular degeneration and in patients with mobility problems in which ultrasound biometry still has a role.

In conclusion, the IOL master is quick and easy to use and provides a noncontact technique with no risk of infection or corneal abrasion and most accepted by patients. It allows accurate AXL measurement and determination of IOL power for cataract surgery.

However, this study is limited by the relatively small number of patients and the need for deeper analysis in SO-filled eyes. This study may be a nidus for future research.